1. Field of the Invention
The present invention pertains to flexible films with electrical parts like traces mounted thereon. Particularly, the film of the present invention can be used in the manufacturing of a novel mechanically variable resistor for use as a rotary position sensor.
2. Description of Related Art
Flexible films have been known for years. A common use of flexible films is to use them as carriers for electrical circuits. A flexible film electrical circuit can consist of conductors only or it can include other electrical components such as resistors.
One use of flexible films is as a component in mechanically variable resistors. Mechanically variable resistors have been produced in a variety of configurations and have innumerable applications. One significant use of variable resistors is as a component in a manufacturing sensor, like a position sensor. Position sensors may be used to determine the relative location of an item that is being moved.
As in the electronics industry, there has been a continuing trend toward smaller sensors that deliver the same or better performance than larger predecessors. However, in the prior art to date, there are factors that can limit the minimum achievable size of variable resistor type position sensors. The limitations are particularly acute where flexible elements are incorporated into the package.
Many prior art resistors incorporate flexible elements. Typically, the flexible elements are located within a housing or other structure. An example of the prior art package designs is given in U.S. Pat. No. 4,355,293, incorporated herein by reference. Many other patents, including U.S. Pat. Nos. 4,430,634 and 4,435,691, incorporated herein by reference, illustrate the use of flexible film elements. Each of these prior art patents illustrate an element that is flexible and retained in position by the package and termination. The element is wrapped in a generally cylindrical form, concentric about the rotor. The element may circle partly around, completely around, or more than 360 degrees around the rotor.
Somewhere within the sensor structure, electrical terminations are mounted to the film. It was common in the prior art to place terminations at both ends of the flexible film element; however, putting terminations at both ends has proven disadvantageous because the electrical connection must still be routed to bring terminations together (from both ends). This requires an expensive complex structure, and furthermore, temperature increases can result in undue stresses in the film and terminations from differential expansion of the various materials. Such forces are undesirable and may lead to immediate disconnection or failures from cyclical stresses over time.
While routing the electrical connections is still practiced using the technique that U.S. Pat. No. 4,355,293 illustrates, it is now more typical to do the routing directly on the flexible element rather than using the housing structure. If the flexible element is a film material such as polyimide, polyester or similar material, it is not difficult to do the electrical routing on the film. A film such as Kapton.TM. is a typical suitable product for use as the flexible element.
Other patents relating to flexible elements and sensors that are herein incorporated by reference for pertinent and supporting information include:
U.S. Pat. No. 3,457,537, describes a flexible resistor element film.
U.S. Pat. No. 5,133,321, discloses an integrated throttle control and idle valve sensor.
U.S. Pat. No. 5,309,134, teaches of a flexible element sensor terminated at an angle off the axis of the rotor.
U.S. Pat. No. 5,385,068, describes an accelerator pedal with a flexible element sensor.
3. Problems with Related Art
Electrical traces mounted on a flexible film element do not tolerate tensional stresses, but can tolerate compressive stresses. As the flexible film element is wrapped about the rotor, the resistor and conductor traces are typically in a compressed state on the inside portion of the film. As shown in Related Art FIG. 1, a flexible film 10 is bent so that it puts the traces (not shown) in the detrimental tension state. Such a bend may be required to place terminations 20 at the desired location as illustrated by bend 36. Placing the resistor or conductor traces in tension can cause cracking that leads to failure of the sensor.
The problem is particularly acute when the sensor design requires terminations 20 to be placed at a location behind the main portion of the flexible film element 10 as shown in FIG. 2. In this instance, bend 36 is very abrupt and thus creates even greater tensional stress in the surface mounted traces (not shown).
It is noted that the above described problem, and other problems are solved through the subject invention and will become more apparent to one skilled in the art, from the detailed description of the subject invention.